Ferrite beads/clamps roughly act as a transformer where the primary is the cable or wire in the clamp and the secondary is shorted. The end result is that high-frequency current is converted into heat. Since the high-frequency (EMI) currents are fairly low unlike 50/60Hz mains currents, the amount of this heat is minuscule. Absolute majority of ferrite materials are targeting high-frequency spectrum (tens of MHz and up). They don't address low-frequency (tens of kHz to few MHz) conducted noise that is prevalent on AC power. The main purpose of ferrite beads/clamps is to reduce radiated emission (30MHz and up) for EMC test - any cable is an antenna and these ferrites introduce losses making what would be a good antenna a bad antenna. One should expect perhaps 10dB attenuation which may be just enough to pass CE/FCC radiated emission test. If you are looking to reduce conducted noise on AC mains, ferrites would be of very little help - you would need to have an AC EMI filter for that.